1. Field of the Invention
The present invention relates to the implementation of a method and apparatus for estimating battery charge power and discharge power.
2. Background Art
A number of high-performance battery applications require precise real-time estimates of the power available to be sourced by the battery pack. For example, in Hybrid Electric. Vehicles (HEVs) and Battery Electric Vehicles (BEVs), the vehicle controller requires continuous up-to-date information from the Battery Management System (BMS) regarding the power that may be supplied to the electric motor from the battery pack, and power that may be supplied to the pack via regenerative braking or by active recharging via the motor. One current technique in the art, called the HPPC (Hybrid Pulse Power Characterization) method, performs this task of estimation by using the voltage limits to calculate the maximum charge and discharge limits. As described in the PNGV (Partnership for New Generation Vehicles) Battery Test Manual, Revision 3, February 2001, published by the Idaho National Engineering and Environmental Laboratory of the U.S. Department of Energy, the HPPC method estimates maximum cell power by considering only operational design limits on voltage. It does not consider design limits on current, power, or the battery state-of-charge (SOC). Also the method produces a crude prediction for horizon Δt. Each cell in the battery pack is modeled by the approximate relationshipvk(t)=OCV (zk(t))−R×ik(t)  (1)where OCV (zk(t)) is the open-circuit-voltage of cell k at its present state-of-charge zk(t) and R is a constant representing the cell's internal resistance. Different values of R may be used for charge and discharge currents, if desired, and are denoted as Rchg and Rdis, respectively.
Since the design limits vmin≦vk (t)≦vmax must be enforced, the maximum discharge current may be calculated as constrained by voltage, as shown below
                              i                      max            ,            k                                dis            ,            volt                          =                                                            OCV                ⁡                                  (                                                            z                      k                                        ⁡                                          (                      t                      )                                                        )                                            -                              υ                min                                                    R              dis                                .                                    (        2        )            The maximum magnitude charge current may be similarly calculated based on voltage. Note, however, that charge current is assumed negative in sign by convention employed in the present invention (although the opposite convention may be used with minor modifications to the method), so that maximum-magnitude current is a minimum in the signed sense. It is
                              i                      min            ,            k                                chg            ,            volt                          =                                                            OCV                ⁡                                  (                                                            z                      k                                        ⁡                                          (                      t                      )                                                        )                                            -                              υ                max                                                    R              chg                                .                                    (        3        )            Pack power is then calculated as
                              p          min          chg                =                              n            s                    ⁢                      n            p                    ⁢                                    max              k                        ⁢                          (                                                υ                  max                                ⁢                                  i                                      min                    ,                    k                                                        chg                    ,                    volt                                                              )                                                                        p          max          dis                =                              n            s                    ⁢                      n            p                    ⁢                                    min              k                        ⁢                                          (                                                      υ                    min                                    ⁢                                      i                                          max                      ,                      k                                                              dis                      ,                      volt                                                                      )                            .                                          
This prior art charge calculation method is limited in several respects. First, as noted above, the method does not use operational design limits on SOC, maximum current, or maximum power in the computation. More importantly, the cell model used is too primitive to give precise results. Overly optimistic or pessimistic values could be generated, either posing a safety or battery-health hazard or causing inefficient battery use.
What is desired is a new method and appartus for battery charge estimation based on a better cell model. Such a cell model would be combined with a maximum-power algorithm that uses the cell model to give better power prediction. The new method would also take in operational design limits such as SOC, current, and power.